Data transmission methods with payload unit numbering in a telecommunication system

ABSTRACT

The invention relates to data transmission in telecommunication systems and particularly in radio systems. The invention employs “payload numbering” instead of or in addition to conventional frame numbering. Data ( 61 ) is split into fixed-length data blocks or payload units ( 62 ). The size of a block is preferably equal to or smaller than the shortest information field in frames ( 63 ) of the protocol(s) used. Each protocol frame carries one or more payload units. In an optimum situation the length of the information field in a protocol frame equals n * the length of the payload unit, where n is an integer. Instead of frame numbering (in some special cases possibly in addition to it) the protocol frame carries payload numbers both for indicating the payload units (data blocks) conveyed in the protocol frame and for acknowledging the received blocks.

This application is the national phase of international applicationPCT/FI99/00477 filed Jun. 1, 1999 which designated the U.S., and theinternational application was published under PCT Article 21(2) inEnglish.

The invention relates to data transmission in telecommunication systemsand particularly in radio systems.

Mobile communication systems generally refer to differenttelecommunication systems which enable personal wireless datatransmission while subscribers roam in the system area. A typical mobilecommunication system is a Public Land Mobile Network (PLMN).First-generation mobile communication systems were analogue systemswhere speech or data was transferred in an analogue form similarly as inconventional public switched telephone networks. An example of afirst-generation system is the Nordic Mobile Telephone (NMT).

In second-generation mobile systems, such as the Global System forMobile Communication (GSM), speech and data are transmitted in a digitalform. In addition to conventional speech transmission, digital mobilecommunication systems provide a plurality of other services: shortmessages, facsimile, data transmission, etc.

Currently under development are third-generation mobile communicationsystems, such as the Universal Mobile Communication System (UMTS) andthe Future Public Land Mobile Telecommunication System (FPLMTS), whichwas later renamed as the International Mobile Telecommunication 2000(IMT-2000). The UMTS is being standardized by the EuropeanTelecommunication Standards Institute (ETSI), whereas the InternationalTelecommunication Union (ITU) standardizes the IMT-2000 system. Thesefuture systems are basically very similar. For example the UMTS, as allmobile communication systems, provides wireless data transmissionservices to mobile subscribers. The system supports roaming, which meansthat UMTS users can be reached and they can make calls anywhere as longas they are situated within the coverage area of the UMTS.

Services provided by mobile communication systems can generally bedivided into teleservices and bearer services. A bearer service is atelecommunication service which forms signal transmission betweenuser-network interfaces. For example modem services are bearer services.In a teleservice the network also provides subscriber terminal services.Important teleservices include speech, facsimile and videotex services.Bearer services are usually divided into groups according to a property,such as asynchronous and synchronous bearer services. Each of thesegroups comprises a number of bearer services, such as a transparentservice (T) and a non-transparent service (NT). In a transparent servicethe (real-time) data to be transmitted, is unstructured and transmissionerrors are corrected only by means of channel coding. In anon-transparent service the (non-real-time) data to be transmitted isstructured into protocol data units (PDU) and transmission errors arecorrected by utilizing (in addition to channel coding) automaticretransmission protocols, i.e. retransmission of corrupted data in thedata link layer. For example in the GSM system such a link protocol iscalled a radio link protocol (RLP). This kind of link protocol isalso-generally referred to as link access control (LAC), particularly inconnection with third-generation mobile communication systems.

In a retransmission protocol the data is transmitted in frames (dataunits), which usually comprise a frame check sequence FCS, which iscalculated on the basis of the frame content. The receiver checks thecontent of the received frames by calculating the FCS on the basis ofthe content of the received frame and by comparing it with the FCSreceived in the frame. If the FCSs do not match, the frame isinterpreted as corrupted and the receiver requests for retransmission ofthe frame. The receiver also requests for retransmission when the frameis missing entirely. In such a manner the radio system is able toprovide a user with a data channel which has a better bit error ratio(BER) than a data channel with no retransmission protocol in use. Forexample in the GSM system, the basic BER (without RLP) is usually about10⁻³, whereas with the use of RLP the BER is about 10⁻⁸. On the otherhand, the effective data rate naturally deteriorates due to numerousretransmissions.

The retransmission protocol retransmits the entire frame whenever theFCS calculated in the receiver does not match the received FCS. This mayresult from an error of one bit in the frame. This speaks for the use ofshort frames in order that the amount of data that is possiblyretransmitted as a result of bit errors can be minimized. On the otherhand, each frame has a header, which contains sequence numbers foridentifying the frame, and an FCS field. This overhead, in turn, speaksfor longer frames in order that the overhead in the frames can beminimized. The longer the frames, the smaller the portion of theoverhead with respect to the amount of the data to be transmitted. Atpresent there are protocols which employ fixed-length data frames (suchas the GSM RLP) and protocols utilizing variable-length protocols, suchas Logic Link Control LLC in the packet data service GPRS of the GSMsystem.

Third-generation mobile communication systems may requirevariable-length frames of the data link layer for different reasons, forexample in order to achieve optimum adaptation to changing conditions ofan underlying Medium Access Control (MAC) layer and to varying radioconditions. In third-generation systems it is possible to use differentMAC services with different BERs from about 10⁻³ to 10⁻⁶ with or withoutMAC layer retransmission. However, there is a problem related toadaptive changing of the frame length.

If the radio conditions deteriorate, the frame length is made shorter.The shorter the frame, the less susceptible it is to disturbance and thegreater the likelihood that the frame is transmitted over the radio pathwithout distortion. On the other hand, if the frames are very long eachframe is subjected to bit errors during the transmission and thetransmission only consists of retransmissions. When the frame lengthchanges during the connection, it is likely that the transmissionbuffers will contain long frames waiting for retransmission. However,such long frames cannot be divided into several short frames since thiswould make the frame numbering meaningless and thus prevent correctoperation. In other words, if long frames that have already beentransmitted are retransmitted in short frames with different framenumbers, it will confuse the complicated sequences of retransmissionsand retransmission requests, possibly resulting in loss or doubling ofdata. Therefore long frames must be retransmitted even if the optimumframe length used by the new frames may be considerably shorter.

Transfer to the use of third-generation mobile communication systemswill take place gradually. At the beginning, third-generation radioaccess networks will be used in connection with network infrastructureof second-generation mobile communication systems. Such a hybrid systemis illustrated in FIG. 1. A second-generation mobile services switchingcentre MSC is connected both to a second-generation radio accessnetwork, such as a GSM base station system BSS consisting of a basestation controller BSC and base stations BTS, and to a third-generationradio access network consisting of, for example, a radio networkcontroller RNC and base stations BS. In practice, there will be twodifferent radio subsystems RSS, which share a common infrastructure onthe network subsystem NSS level. Second-generation mobile stations MS(such as the GSM) communicate via the second-generation radio accessnetwork and third-generation mobile stations MS (such as the UMTS)communicate via the third-generation radio access network. Possibledual-mode phones (such as GSM/UMTS) are able to use either radio accessnetwork and to perform handovers between them.

Since a third-generation radio access network has not been designed tobe compatible with a second-generation core network (NSS), it is clearthat such a mixed architecture requires interworking between thenetworks, usually described in the form of an interworking unit IWU. Ageneral requirement is that no modifications are allowed in thesecond-generation system (mobile services switching centre MSC), whichmeans that the interface connecting, for example, the GSM MSC and theIWU should be a pure A interface. The IWU should carry out all theconversions between the second-generation and third-generation functionsand formats. Since the second-generation and third-generationretransmission protocols (such as RLP and LAC) will be at least somewhatdifferent, interworking which will probably be needed between thesecond-generation and third-generation rip systems is the adaptation ofthese different protocols to each other.

Subsequent development will lead to a situation where purethird-generation mobile communication networks exist in parallel withsecond-generation mobile systems or the aforementioned hybrid systems.FIG. 2 illustrates this situation.

An object in the development of third-generation mobile communicationsystems is supporting a handover between second-generation andthird-generation mobile systems. A dual-mode mobile station should beable to roam from a second-generation radio access network to athird-generation radio access network and vice versa without a break inan ongoing call.

This object can be reached rather easily for speech calls or transparentdata calls. A handover causes loss or doubling of only a few bits whentraffic channel protocol stacks are being swapped. Speech does notrequire amendment of these few bit errors, since they only cause amomentary disturbance or no noticeable change in the received speech. Intransparent data transmission, end-to-end application protocols correctsingle bit errors.

The situation is different when a handover is carried out fornontransparent data calls. As stated above, NT calls utilize aretransmitting link protocol, such as RLP or LAC, (in addition tochannel coding) for error correction. Second-generation andthird-generation protocols will be at least somewhat different.Therefore it is necessary to change the link protocol during a handover.However, in a handover there may be complicated ongoing sequences ofselective retransmissions and retransmission requests in the “old” linkprotocol, and an interruption thereof possibly leads to loss or doublingof data. Yet, in order to maintain data integrity it is important thatnot one bit is lost or doubled during the swap of the traffic channelprotocol stacks.

An object of the invention is to eliminate the problems related to theretransmission of old frames when the frame length of the retransmissionprotocol is changed during the connection.

Another object of the invention is the interworking between link layerprotocols of different radio systems.

Yet another object of the invention is to develop a handover methodwhich maintains data integrity in a handover for a non-transparent callbetween two mobile communication systems.

The basic idea of the invention is to utilize “payload unit numbering”instead of or in addition to conventional frame numbering. The data issplit into fixed-length data blocks or payload units. The size of ablock is preferably equal to or smaller than the shortest informationfield in the frames of the protocol(s) used. Each protocol frame carriesone or more payload units. In an optimum situation the length of theinformation field in a protocol frame equals n * the length of thepayload unit, where n is an integer. Instead of (in some special casespossibly in addition to) frame numbering the protocol frame carriespayload numbers both for indicating the payload units (data blocks)transferred in the protocol frame and for acknowledgement of thereceived blocks.

The payload numbering according to the invention is thus based on thenumbering of the units formed from the data content, wherefore it isindependent of the frame length and the frame type, i.e. the usedprotocol. This provides considerable advantages.

By means of the payload numbering it is possible to avoid theaforementioned problems related to changing the length of the protocolframe. After the frame length has changed, the transmitter splits the“old” frames in the retransmission buffer back into payload units, packsthese payload units into “new” frames and indicates with payloadnumbering in the header of the new frame which payload units the framecontains (e.g. by indicating the number of the first payload unit in theframe). The receiver identifies the change in the frame length (e.g.from the frame header) and the payload numbers (both the numbers sentfor identification of the received frames and the numbers sent foracknowledgement) from the frame header similarly as before the change ofthe frame length. The retransmission sequences are not disturbed as aresult of the change in the frame length since the payload numbering isthe same as before the change. The only thing that changes is the framecapacity, i.e. the number of payload units transferred in one frame.Therefore the invention optimizes the performance of a non-transparentdata traffic channel under varying radio and error conditions.

By means of the payload unit numbering it is also possible to improvethe interworking of link layer protocols between two different radiosystems. The length of the payload unit can be selected optimallyregarding the protocols used by both systems. For example, the length ofthe payload unit can be negotiated at the start of each connection inthe same way as all the other link layer parameters, or the length canbe indicated directly or indirectly in connection with signalling, orthe length can be fixed. In an embodiment of the invention, a radioaccess network (e.g. a third-generation radio access network) where theprotocol (e.g. LAC) allows changing the frame length is connected toanother radio system (e.g. a second-generation radio system) where theprotocol (e.g. RLP) frame has a fixed length. The length of the payloadunit can be selected to be identical to the length of the informationfield in the RLP frame, in which case each RLP frame carries one payloadunit and the payload numbering is directly compatible with the RLP framenumbering. Therefore the same numbering applies over the entireconnection for example between the mobile station and the mobileservices switching centre even though the connection comprises two legswith different layer 2 link protocols and even different frame lengths.This simplifies the implementation of the interworking between thesystems since the interworking does not have to adapt two differentframe numbering systems to each other, but it only attends to theadaptation of the different protocol functions and formats and to thetransmission of information (user data and protocol commands andresponses). If either protocol does not support a particular protocolfunction, the interworking unit can deactivate the function for exampleby means of negative acknowledgement when the link parameters are beingnegotiated at the beginning of the connection. Also, the same numberingfrom end to end enables handovers without loss or doubling of data.Alternatively, the length of the payload unit can be selected such thatthe frame of the first protocol (e.g. third-generation LAC) can betransmitted through a channel of the second radio system instead of thesecond protocol (e.g. RLP) frame or in the information field thereof. Inthis case, too, the same numbering is applied from end to end, whichprovides several advantages. The invention also makes it possible tochange the frame length at the radio interface in steps of a payloadunit even though the frame length at the network interface between themobile services switching centre and the interworking unit stays thesame. Therefore the frame length at the radio interface can be adaptedto radio conditions, error conditions, etc.

The payload units are numbered preferably in ascending order. Therefore,under normal conditions it is sufficient that the payload units in theframe are identified with one payload unit number (e.g. the number ofthe first payload unit). In such a case the invention causes no or onlyminimum overhead. A different situation occurs when the data rate ischanged from a lower to a higher rate or vice versa, and the higher datarate is not an even multiple of the lower data rate. In such a casethere may be scattered payload units (which are no longer in theoriginal sequence), which must be retransmitted. According to anembodiment of the invention, in these less than optimum situations thepayload unit number is indicated in the frame separately for eachpayload unit by means of so-called header extension. This means atemporary increase in overhead. However, these situations are rare sinceonly about 5 to 10% of the frames are assumed to be retransmitted andonly a fraction of these will be out of sequence and subject to changesof data rate.

The invention will be described below in greater detail in connectionwith preferred embodiments, with reference to the accompanying drawings,in which

FIG. 1 shows a second-generation mobile communication networksupplemented with a third-generation radio access network;

FIG. 2 shows a second-generation and a third-generation network betweenwhich dual-mode mobile stations can roam;

FIG. 3 shows a protocol stack of a non-transparent data service in theGSM system;

FIG. 4 shows protocol layers of a third-generation mobile system inanother manner;

FIG. 5 illustrates a basic structure of an LAC frame;

FIGS. 6A to 6C illustrate payload unit numbering according to theinvention;

FIG. 7 illustrates data transmission and retransmission based on payloadnumbering;

FIGS. 8A, 8B, 8C and 9 illustrate the retransmission according to theinvention when the frame length changes;

FIG. 10 shows a mobile communication system where a third-generationradio access network is connected to a second-generation mobile servicesswitching centre;

FIG. 11 shows data transmission in the system of FIG. 10 when payloadunit numbering is used,

FIG. 12 illustrates a handover according to the invention during thetransmission of an LAC frame;

FIG. 13 illustrates an extendible header of a protocol frame; and

FIG. 14 shows a frame where the header comprises individual numbers offour payload units.

The present invention can be applied in any telecommunication systemwith a link protocol frame of variable length, or in interworking orhandover between any two digital radio systems with different radio linkprotocols. “Radio system” should be understood broadly such thatdifferent radio access networks of the same mobile network are able toform different radio systems, as illustrated in FIG. 1, or that radiosystems refer to entirely separate mobile communication systems, asshown in FIG. 2. One or both of the radio access networks can bewireless local loop (WLL) or radio local loop (RLL) networks. Theprimary field of application of the invention is a handover between asecond-generation and a third-generation mobile network, such as the GSMand the UMTS. “Link protocol” should also be understood herein generallyto cover not only the present second-generation protocols, such as theRLP of the GSM system, but also all the possible third-generation orlater generation link access control (LAC) protocols or the radio linkcontrol protocol (RLCP) of the Wideband CDMA system, or also lower-layerretransmission protocols, such as the medium access control (MAC). Inthe following, the preferred embodiments of the invention will bedescribed by using as an example the second-generation GSM system andthe third-generation UMTS. In the description below, the GSM radio linkprotocol will be called RLP and the UMTS radio link protocol will becalled LAC.

A GSM network consists of two basic parts: a base station system BSS anda network subsystem NSS. The BSS and the mobile stations MS communicatevia radio connections. In the BSS, each cell is served by a base stationBTS. A number of BTSs are connected to a base station controller BSC thefunction of which is to control radio frequencies and channels used by aBTS. The BSCs are connected to a mobile services switching centre MSC.Certain MSCs are connected to other telecommunication networks, such asthe public switched telephone network PSTN, and they comprise gatewayfunctions for calls terminating at and originating from these networks.These MSCs are known as gateway-MSCs (GMSC). There are also at least twodatabases: a home location register HLR and a visitor location registerVLR.

A mobile communication system comprises adapter functions for adaptingan intra-network data link to the protocols used by terminal equipmentsand other telecommunication networks. The adapter functions typicallyinclude a terminal adaptation function TAF placed at the interfacebetween a mobile station and a data terminal equipment connectedthereto, and an interworking function IWF situated at the interfacebetween the mobile and network and another telecommunication network,usually in connection with an MSC. In the GSM system a data link is setup between a TAF of the MS and an IWF in the mobile network. The TAFadapts a data terminal equipment DTE connected or integrated to the MSto the aforementioned GSM data link that is set up over a physicalconnection using one or several traffic channels. The IWF connects theGSM data link for example to another telecommunication network, such asan ISDN or another GSM network, or to some other transit network, suchas a PSTN.

FIG. 3 illustrates protocols and functions required for nontransparentbearer services. A non-transparent circuit switched connection between aTAF and an IWF on a GSM traffic channel comprises several protocollayers that are common to all these services. They include differentrate adaptation RA functions, such as RA1′ between the TAF and a channelcodec unit CCU placed in the BSS, RA1 between the CCU and the IWF, RAA(or RAA′ for a 14.4 kbit/s channel) between the CCU and a transcoderunit TRAU placed remote from the base station, and RA2 between the TRAUand the IWF. The rate adaptation functions RA are defined in the GSMrecommendations 04.21 and 08.20. Communication between the CCU and theTRAU is defined in the GSM recommendation 08.60. Information that hasbeen RA1′ rate-adapted in the radio interface is also channel-coded asdefined in the GSM recommendation 5.03, which is illustrated by blocksFEC in the MS and the CCU. The IWF and the TAF also comprisehigher-level protocols that are specific to each service. In anasynchronous non-transparent bearer service the IWF requires an L2R(Layer 2 Relay) protocol and a radio link protocol RLP and a modem or arate adapter towards the fixed network. The L2R functionality fornon-transparent character oriented protocols is defined for example inthe GSM recommendation 07.02. The RLP is defined in the GSMrecommendation 04.22. The RLP is a frame-structured, balanced(HDLC-type) data transmission protocol, where error correction is basedon retransmission of distorted frames at the request of the receivingparty. The interface between the IWF and for example an audio modemMODEM is as defined in CCITT V.24 and it is denoted in FIG. 3 by L2.This non-transparent configuration is also used to access the Internet.

The RA1 and RA1′ rate adaptations map each 240-bit RLP frame into fourmodified 80-bit V.110 frames (between the MSC and the BSS) or into fourmodified 60-bit V.110 frames (at the radio interface). A bit sequencecalled “Frame Start Identifier” is used to indicate which V.110 frame inthe bit stream is the first one for a particular RLP frame. The firstquarter of the RLP frame is transmitted in this V.110 frame, the secondquarter is transmitted in the next frame, the third quarter in the thirdframe and the fourth one in the fourth frame, whereafter a new RLP framebegins.

In the HSCSD concept of the GSM system, a high-speed data signal isdivided into separate data streams, which are then transmitted via Nsubchannels (N traffic channel time slots) at the radio interface. Whenthe data streams have been divided they are conveyed in the subchannelsas if they were mutually independent until they are again combined inthe IWF or the MS. However, logically these N subchannels belong to thesame HSCSD connection, i.e. they form one HSCSD traffic channel.According to the GSM recommendations dividing and combining a datastream are carried out in a modified RLP, which is thus common to allthe subchannels. Below this common RLP each subchannel comprisesseparately the same protocol stackRA1′-FEC-FEC-RA1′-RAA-RAA-RA2-RA2-RA1, which is shown in FIG. 3 for onetraffic channel between the MS/TAF and the MSC/IWF. Therefore an HSCSDtraffic channel according to the GSM recommendations will still use thecommon RLP for the different subchannels even though the bit rate on asingle subchannel can be as high as 64 kbit/s.

An example of a third-generation network is the UMTS, which is stillunder development. It should be noted that the detailed structure of theUMTS access network is not significant for the invention. According tothe simplest scenario the UMTS is an access network the functions ofwhich are strictly limited to radio access functions. Therefore itmainly comprises functions for controlling radio resources (handover,paging) and for controlling bearer services (radio network servicecontrol). The more complicated functions, such as registers, registerfunctions, mobility management and location management, are placed in aseparate network subsystem NSS or in the core network. The NSS or thecore network may be, for example, the GSM infrastructure. In FIGS. 1 and2 the third-generation radio access network comprises base stations BSand a radio network controller RNC. It is further assumed that thethird-generation system employs the link access control LAC protocolbetween the MS and the MSC/IWF, the protocol differing fromsecond-generation radio link protocols, such as the RLP. A physicaltraffic channel comprises lower protocols, in the frames of which theLAC frames are transmitted. In principle a protocol stack of athird-generation mobile communication system may be similar as describedabove in connection with the GSM system, except that RLP is replacedwith LAC.

FIG. 4 illustrates in another manner protocol layers of a purethird-generation mobile communication system. The LAC protocol extendsfrom end to end between an MS and an MSC. At the radio interface betweenthe MS and the BS/RNC there is MAC (Medium Access Control) and aphysical layer (radio channel) below the LAC. At the network interfacebetween the BS/RNC and the MSC there is a transmission layer and aphysical layer (transmission channel) below the LAC. FIG. 5 shows abasic structure of an LAC frame comprising a fixed-length header, avariable-length information field, and a fixed-length frame checksequence FCS. It is possible that in third-generation systems the LACthroughput is optimized under varying radio conditions by manipulatingthe length of the LAC frame. Generally, there can be two reasons forchanging conditions: different radio environments and different MACbearer services. At the start of a connection it is possible to use foroptimum frame size a default value based on connection parameters.During the connection it is possible to monitor the quality of the datatransmission, for example the frame error ratio (FER). If the FER dropsbelow a predetermined limit indicating good conditions, the frame sizeis increased. If the FER exceeds another predetermined limit, the framesize is decreased. With such an arrangement the LAC tries to optimizethe frame size for the radio conditions and bit error ratio in eachcase. However, there may be specified maximum and minimum values for theframe size, depending on the bit rate. If the data is not transmittedsufficiently rapidly for some reason, the actual frame size can besmaller than the optimum frame size in order to avoid delays. The MAClayer can also indicate the present conditions, thus helping the LAC toadapt more rapidly. The optimum frame size can be the same or differentfor different transmission directions, which means that both ends areable to negotiate the optimum frame size or each end uses its ownoptimum frame size. It should be noted that the arrangement describedabove is only a scenario of the inventors concerning the adjustment ofthe frame length. It is not essential to the invention how the framelength is changed. The invention can also be applied in situations wherethe frame length is fixed or agreed upon only at the start of theconnection.

FIGS. 6A to 6C illustrate the payload unit numbering according to theinvention. A transmitter splits a data stream 61 to be transmitted intofixed-length data blocks or payload units 62. The size of the payloadunit 62 is preferably equal to or smaller than the shortest informationfield in the frames of the used protocol(s), such as the LAC. Thetransmitter and/or the receiver obtains the length of the payload unitdirectly or indirectly from outband or inband signalling. The length canalso be negotiated at the beginning of the connection or again duringthe connection. The payload units 62 are inserted into the informationfield of LAC frames 63. Therefore each LAC frame 63 carries one or morepayload units 62. In an optimum situation the length of the informationfield in the LAC frame 63 equals n * the length of the payload unit 62,wherein n is an integer. For example in FIG. 6C, the LAC frames comprisen payload units. Each LAC frame 63 further comprises a frame checksequence FCS. Instead of frame numbering the LAC frame carries in theheader field H the payload unit numbering indicating which payload unitsare transferred in the information field of the LAC frame. In theexample shown in FIG. 6C, the numbering in the header field, i.e. aso-called transmission number, indicates that in addition to the numberof the first payload unit 62, e.g. number 1, the header of the LAC framecan contain the data that the information field of the LAC framecomprises n payload units. The receiver can also conclude itself thenumber of the payload units in the frame, it can know the number inadvance or it can receive information in some other manner. On the basisof the transmission number and the information on the number of payloadunits, the receiver is able to calculate the numbers of the otherpayload units in the frame, if necessary, and the number of the nextpayload unit it wants. The receiver can transmit this next number, i.e.a so-called reception number, in acknowledgement to the transmitter ifthe LAC frame was received successfully. As a result of theacknowledgement the transmitter transmits the requested payload unit andn−1 next payload units in the next LAC frame. If the FCS indicates thatthe content of the received LAC frame was faulty or if the frame ismissing entirely, the receiver can request for retransmission of theentire LAC frame by sending in acknowledgement the transmission numbergiven in the faulty frame. If it can be concluded from the FCS that thefaulty bit is in the kth payload unit (where k is an integer and ken),the receiver can transmit the number of this corrupted payload unit inacknowledgement in an embodiment of the invention. As a result of theacknowledgement the transmitter retransmits the requested payload unitand (n−k+1) following payload units together with (k−1) new payloadunits in the next LAC frame. If the data transmission is bidirectional,the above-described operation can take place in both transmissiondirections. In such a case the header H of the LAC frames 63 may containboth a transmission number for one transmission direction and areception number for the other transmission direction. Further, it ispossible to use windowing with the payload numbers similarly as in theprotocols based on frame numbers.

FIG. 7 shows an example of data transmission and retransmission based onthe payload numbering according to the invention. A transmitter Txtransmits an LAC frame 71, which comprises three payload units, numbers1, 2 and 3, and it stores the LAC frame 71 or only the payload units 1to 3 in a retransmission buffer. The header of the LAC frame 71indicates the number of the first payload unit, i.e. number 1. Areceiver Rx receives the LAC frame without errors and transmits an LACacknowledgement frame 72, where the header indicates the number of thenext desired payload unit, i.e. number 4. The transmitter Tx transmitsthe next LAC frame 73, which contains payload units 4, 5 and 6, and itstores the LAC frame 73 or only payload units 4 and 5 in theretransmission buffer. The reception of the entire LAC frame 73 failsand the receiver Rx transmits: an LAC acknowledgement frame 74 wherepayload unit 4 is again requested for. The transmitter Tx retransmitspayload units 4, 5 and 6 in an LAC frame 75,

FIGS. 8A, 8B and 8C illustrate how a transmitter Lx processes frames tobe retransmitted when the frame length is changed during the connection.FIG. 8A shows an “old” frame in the transmission buffer, comprising npayload units. After the frame length has changed, the transmitter Txsplits the “old” frame back into-payload units (FIG. 8B) and packs thepayload numbers into “new” frames, each of which comprises two payloadunits (FIG. 8C). The payload numbering in the header of the new frameindicates which payload units the new frame comprises.

FIG. 9 shows an example of the data transmission and retransmissionaccording to the invention, based on payload numbering, when the framelength changes in the middle of the transmission. LAC frames 71 to 74are transmitted in the same manner as in FIG. 7. After LAC frame 73 hasbeen transmitted, the frame length is shortened such that only twopayload units are transmitted in one new frame instead of the old threeunits. After the frame length has changed, the transmitter Tx receivesan acknowledgement frame 74 requesting for retransmission of payloadunits 4 to 6. The transmitter Tx unpacks the old LAC frame 73 as shownin FIG. 8 and inserts payload units 4 and 5 into a new LAC frame 91,which is transmitted to the receiver Rx. The receiver Rx acknowledgeswith an LAC-frame 92, where payload unit 6 is requested for next. Thetransmitter Tx transmits an LAC frame 93 containing the payload unit 6to be retransmitted and a new payload unit 7. The retransmissions couldthus be carried out without interference to the retransmission sequencesas a result of the changed LAC frame length, since the payload numberingand the modes of the transmitter and the receiver remain the same afterthe change. The only thing that changes is the LAC frame throughput,i.e. the number of the payload units transferred in one LAC frame.

By means of the payload numbering it is also possible to improve theinterworking of link layer protocols between two different radiosystems. FIG. 10 shows a mobile communication system where athird-generation radio access network is connected to asecond-generation mobile services switching centre MSC. The radio accessnetwork supports the link access control LAC protocol and the MSCsupports the radio link protocol RLP. Between the radio access networkand the MSC there is an interworking function described in the form ofan interworking unit IWU. The LAC protocol is applied between the MS andthe IWU. The RLP protocol is applied between the IWU and the MSC. TheIWU comprises an LAC/RLP function which understands both the LAC and theRLP formats and converts the transmission formats and functions betweenthe LAC and the RLP. If a particular function is supported by only oneof the protocols, the IWU preferably deactivates such a function duringthe protocol negotiations. Therefore all the functions operate from endto end between the MS and the IWU.

According to the invention, the LAC frames carry data in fixed lengthpayload units, as described above. Also, the retransmission mechanismbetween the MS and the IWU is based on payload numbering and not on LACframe numbering. The length of a payload unit equals the length of aninformation field in an RLP frame. This means that one RLP frame carriesone payload unit. When the retransmission mechanism between the IWU andthe MSC employs conventional RLP numbering, the payload numbering isdirectly compatible with the RLP frame numbering. Therefore the samenumbering applies-over the entire connection between the MS and the MSCdespite two different protocols. In other words, the RLP and the LACprocess the same sequence numbers (the sequence numbers aresynchronized) even though the LAC frames can be longer than the RLPframes. The IWU does not independently acknowledge the data it hasreceived from the MS or the MSC, but it only performs a formatconversion and forwards the information to the receiver, regardless ofwhether the information is user data, acknowledgements or protocolcommands/responses.

FIG. 11 illustrates the data transmission according to the invention ina network configuration of the type shown in FIG. 10. An MS transmits anLAC frame 111 comprising three payload units, numbers 1 to 3. The headerof the frame 111 indicates that the first payload unit is number 1. TheIWU receives the LAC frame 111, unpacks payload units 1 to 3 from theframe 111 and packs them into three RLP frames 112, 113 and 114, theframe numbers of which are correspondingly 1, 2 and 3. The RLP framesare inserted into a transmission buffer. The IWU transmits the first RLPframe (frame number 1) to the MSC. The MSC acknowledges successfulreception with an RLP frame 115, where frame number 2 is requested fornext. Since such a frame is found in the transmission buffer, the IWUtransmits a second RLP frame (frame number 2) to the MSC. The MSCacknowledges successful reception with an RLP frame 116, where framenumber 3 is requested for next. Since such a frame is found in thetransmission buffer, the IWU transmits a third RLP frame (frame number3) to the MSC. The MSC acknowledges successful reception with an RLPframe 117, where frame number 4 is requested for next. This exchange ofRLP frames is shown without the use of windowing, but the changes itcauses in the operation are evident to those skilled in the art. In thewindowing the IWU transmits for example all the RLP frames 112 to 114one after another (the size of a window is three or more RLP frames) andthe MSC only transmits one RLP acknowledgement frame 117. Since framenumber 4 is not found in the transmission buffer, the IWU converts theRLP acknowledgement frame 117 into an LAC acknowledgement frame 118,where the header contains a request for payload unit 4. The MS transmitsa new LAC frame where the information field contains payload units 4 to6 and the header contains payload unit number 4. The IWU stores the RLPframes in the retransmission buffer until it receives acknowledgementfrom the MSC. If, at some point, the MSC transmits an RLP framerequesting for the retransmission of a frame, the IWU transmits therequested frame from its retransmission buffer.

By means of the arrangement according to the invention it is possible toavoid problems in handovers even if the IWU changes since the protocolsstay the same at the connection end points (the MSC and the MS) andthere is no need to reset protocol state machines. Both the MS and theMSC know which frames have already been received and acknowledged.

FIG. 12 shows an example where an MS is handed over from the “old” IWUto the “new” IWU during the transmission of an LAC frame. The startresembles FIG. 11. The MS transmits to the old IWU an LAC frame 121containing three payload units, numbers 1, 2 and 3. The IWU 100 convertsthe LAC frame into RLP format and transmits a first RLP frame 122. TheMSC transmits positive acknowledgement 123 and the IWU transmits asecond RLP frame 124. The MSC also acknowledges this frame (125). Nowthe MS is handed over to a base station which is connected to the “new”IWU 101, and the RLP connection is set up to the new IWU 101. Thereforethe MSC does not receive RLP frame number 3 and the new IWU, 101 doesnot receive acknowledgement for the third RLP frame. Since the MS doesnot receive within a predetermined time acknowledgement for payloadunits 1 to 3 that were transmitted in the LAC frame, an LAC timerexpires and the MS retransmits the payload units in an LAC frame 126 tothe new IWU 101. The new IWU 101 converts the LAC frame 126 into RLPformat and transmits a first RLP frame 127. The sequence number of thefirst RLP frame 127 is the same as the number of the first payload unitin the LAC frame, i.e. 1. The MSC now knows that it has already receivedRLP frames 1 and 2 via the old IWU and it requests with an RLPacknowledgement frame 128 the new IWU to transmit RLP frame number 3.The new IWU 101 transmits an RLP frame 129, i.e. frame number 3, and theMSC transmits an RLP acknowledgement frame 130 where frame number 4 isrequested for next. Since the IWU does not comprise this frame, the IWUconverts the RLP frame 130 into an LAC frame 131; which contains arequest to transmit payload unit 4. The LAC frame 131 thus acknowledgesthe LAC frame 126. The handover was thus carried out without thedoubling or loss of user data.

In a preferred embodiment of the invention, a header of a frame onlycomprises one payload number indicating the first payload unit containedin the frame. This is sufficient and minimizes the overhead under normalcircumstances. However, in special situations, such as when the datarate changes, it may be necessary to indicate in the frame header thenumbers of all the payload units. In the invention this can beimplemented by extending the frame header, as illustrated by examples inFIGS. 13 and 14,

FIG. 13 shows a payload number field of a frame (e.g. an LAC frame),which can be extended. The length of the basic field is 16 bits (octets1 and 2). The payload number (PN) is 14 bits. An H flag indicateswhether the header extension is used or not. If the H flag is 0, theheader ends in this octet and the next two octets (3 and 4) comprisedata (i.e. the beginning of the first payload unit). If the H flag is 1,the next two octets (3 and 4) contain a new payload number. In anembodiment of the invention these additional payload numbers replace thefirst payload unit. A D flag (data segmentation extension) indicateswhether the first payload unit comprises data segmentation information.If the D flag is 1, the first payload unit comprises segmentationinformation. If the D flag is 0, the first data unit comprises data. Thedata is typically divided (segmented) into payload units as a continuousflow and it can also be restored by placing the payload units one afteranother in numerical order. The data segmentation-information isrequired if the data is segmented in an unusual manner.

Normally the payload unit number field only contains octets 1 and 2(H=0) indicating the number of the first payload unit in the frame.

Under special circumstances, for example when the data rate changes, itmay be necessary to retransmit a few payload units “out of sequence” inone or more frames, which means that the payload units in the frames donot have successive numbers. According to an embodiment of theinvention, the number field of the header is then extended by one ormore numbers. This means that in the last number field H=0 and in theother fields H=1. FIG. 14 shows an example of a frame with fourindividually numbered payload units which take the space of five payloadunits. Padding PAD (e.g. zerofill) is used to adapt the frame size intoa multiple of the basic structure. In FIG. 14 the size of the payloadunit is 80 bits, the FCS is 16 bits and the basic header is 16 bits. Theextended header is 16+3*16=64 bits. The padding PAD is 32 bits(80−3*16), wherefore the frame size corresponds to a basic framecomprising four payload units. Normally the frame size changes when thenumber of the payload units is altered, in which case the frame size is16+16+n*80 bits, where n is the number of the payload units. By means ofthe padding PAD also the length of a frame with an extended header is amultiple of the basic frame.

It is obvious that as the technology develops the basic idea of theinvention can be implemented in several different manners. Therefore theinvention and the embodiments thereof are not restricted to the examplesdescribed above, but they may vary within the scope of the claims.

1. A data transmission method in a telecommunication system, the methodcomprising: splitting data to be transmitted into fixed-length payloadunits provided with payload numbers in order to distinguish the payloadunits from one another, inserting one or more payload units into aninformation field of each protocol frame of a link protocol providedwith a retransmission mechanism, providing a header field of a protocolframe with payload numbering, which indicates the payload unitscontained in the information field of the protocol frame, transmittingthe protocol frame from a transmitting end to a receiving end,acknowledging payload units which have been received appropriately,requesting transmission of new payload units or requestingretransmission of payload units which have not been receivedappropriately by means of said payload numbers, using data blocknumbering in said retransmission mechanism, changing the length of theprotocol frame during a connection, and inserting the payload units tobe retransmitted into one or several protocol frames with a new framelength, said payload units having been transmitted for the first timebefore the frame length was changed.
 2. A method according to claim 1,further comprising: unpacking payload units from protocol frames havinga previous frame length and being contained in a retransmission buffer,at the transmitting end after the frame length has been changed.
 3. Amethod according to claim 1, wherein said link protocol provided with aretransmission mechanism is a layer 2 link protocol or a protocolsituated below a layer 2 link protocol.
 4. A method according to claim3, wherein the layer 2 link protocol is a radio link protocol, a linkaccess control protocol or a radio link control protocol.
 5. A methodaccording to claim 3, wherein the protocol situated below a layer 2 linkprotocol is medium access control.
 6. A method according to claim 1,further comprising: indicating in the header of the protocol frame in anormal situation the payload number of only one payload unit containedin the information field, and indicating in the header of the protocolframe the payload number of every payload unit in the information fieldindividually, when payload units with unsuccessive numbers areretransmitted in the protocol frame in a special situation.
 7. A methodaccording to claim 6, further comprising: indicating said individualpayload numbers in a frame header extension at the beginning of saidinformation field.
 8. A method according to claim 6, wherein saidspecial situation is a change in data rate.
 9. A data transmissionmethod in a mobile communication system comprising a mobile servicesswitching centre with a first link protocol having a fixed frame length,the first link protocol provided with a retransmission mechanism fornontransparent data transmission; a radio access network with a secondlink protocol provided with a retransmission mechanism fornon-transparent data transmission, a frame length of the second protocolbeing variable or a frame thereof being longer than a frame of the firstprotocol; and an interworking unit via which the radio access network isconnected to the mobile services switching centre, the methodcomprising: transmitting data in frames of the first link protocolbetween the interworking unit and the mobile services switching centre,using frame numbering in said retransmission mechanism of the first linkprotocol between the interworking unit and the mobile services switchingcentre, transmitting data in frames of the second link protocol betweenthe mobile station and the interworking unit, transmitting data in theinformation fields of the second link protocol frames in the form ofdata blocks which are numbered, the length of each of said data blocksbeing equal to the length of the information field of a first linkprotocol frame, and using said data block numbering in saidretransmission mechanism of the second link protocol between theinterworking unit and the mobile station, said data block numberingbeing directly compatible with the frame numbering used between theinterworking unit and the mobile services switching centre.
 10. A datatransmission method in a mobile communication system comprising a mobileservices switching centre with a first link protocol having a fixedframe length, the first link protocol provided with a retransmissionmechanism for nontransparent data transmission; a radio access networkwith a second link protocol provided with a retransmission mechanism fornon-transparent data transmission, the frame length of the secondprotocol being variable; and an interworking unit via which the radioaccess network is connected to the mobile services switching centre, themethod comprising: transmitting data in frames of the first linkprotocol between the interworking unit and the mobile services switchingcentre, transmitting data in frames of the second link protocol betweenthe mobile station and the interworking unit, transmitting data ininformation fields of the second link protocol frames in the form ofdata blocks which are numbered, selecting a length of a data block suchthat the frame length of the second link protocol is equal to or smallerthan the length of the first link protocol frame or information field,transmitting the frames of the second link protocol in place of theframes of the first link protocol or in the information fields thereofbetween the interworking unit and the mobile services switching centre,and using said data block numbering in the retransmission mechanism ofthe second link protocol over an entire connection between the mobilestation and the mobile services switching centre.
 11. Atelecommunication system comprising a transmitter and a receiver and alink protocol provided with a retransmission mechanism, the transmitterand the receiver being arranged to transmit data in frames of the linkprotocol from a transmitting end to a receiving end, wherein the data isplaced in information fields of the protocol frames in fixed-length datablocks which are numbered, and said retransmission mechanism is arrangedto utilize said data block numbering, and the length of a protocol framecan be changed during a connection, and the transmitter is arranged toinsert payload units to be retransmitted into one or several protocolframes with a new frame length in response to the changing of the framelength, said payload units having been transmitted for the first timebefore the frame length was changed, and wherein the information fieldof each protocol frame comprises one or more data blocks and a headerfield of a protocol frame is provided with payload numbering indicatingthe payload units in the information field of the protocol frame.
 12. Asystem according to claim 11, wherein the receiver is arranged toacknowledge appropriately received payload units, to requesttransmission of new payload units or to request retransmission ofinappropriately received payload units by means of said payload numbers.13. A system according to claim 11, wherein the transmitter is arrangedto unpack payload units from protocol frames having a previous framelength and being contained in a retransmission buffer, in response tothe changing of the frame length.
 14. A system according to claim 11,wherein said link protocol provided with a retransmission mechanism is alayer 2 link protocol or a protocol situated below a layer 2 linkprotocol.
 15. A system according to claim 11, wherein the length of apayload unit can be obtained either directly or indirectly from inbandor outband signalling.
 16. A system according to claim 11, wherein thelength of a payload unit can be negotiated at the beginning of theconnection and/or during the connection.
 17. A system according to claim11, wherein the header of the protocol frame normally contains thepayload number of one payload unit in the information field, and theheader of the protocol frame contains the individual payload number ofeach payload unit in the information field when payload units withunsuccessive numbers are retransmitted in the protocol frame in aspecial situation.
 18. A system according to claim 17, wherein theheader of the protocol frame can be extended to the beginning of theinformation field in order to indicate said individual payload numbers.19. A mobile communication system comprising a mobile services switchingcentre with a first link protocol provided with a fixed frame length anda retransmission mechanism utilizing frame numbering for non-transparentdata transmission; a radio access network with a second link protocolprovided with a retransmission mechanism for non-transparent datatransmission, a frame length of the second protocol being variable or aframe thereof being longer than a frame of the first protocol; and aninterworking unit via which the radio access network is connected to themobile services switching centre such that a transmission link isprovided between a mobile station and the mobile services switchingcentre via the radio access network, the transmission link comprising afirst leg between the interworking unit and the mobile servicesswitching centre and a second: leg between the mobile station and theinterworking unit, wherein data is situated in information fields of thesecond link protocol frames in the form of data blocks which arenumbered, the length of each of said data blocks equalling the length ofan information field of the first link protocol frame, and theretransmission mechanism of the second link protocol is arranged to usesaid data block numbering between the interworking unit and the mobilestation, said data block numbering being directly compatible with theframe numbering used between the interworking unit and the mobileservices switching centre.
 20. A mobile communication system comprisinga mobile services switching centre with a first link protocol having afixed frame length, provided with a retransmission mechanism fornon-transparent data transmission; a radio access network with a secondlink protocol provided with a retransmission mechanism fornon-transparent data transmission, a frame length of the second protocolbeing variable; and an interworking unit via which the radio accessnetwork is connected to the mobile services switching centre such that atransmission link is provided between a mobile station and the mobileservices switching centre via the radio access network, the transmissionlink comprising a first leg between the interworking unit and the mobileservices switching centre and a second leg between the mobile stationand the interworking unit, wherein the mobile station and theinterworking unit are arranged to transmit data in the informationfields of the second link protocol frames in the form of data blockswhich are numbered, and the length of each data block is such that theframe length of the second link protocol is equal to or smaller than alength of the first link protocol frame or information field, and theinterworking unit and the mobile services switching centre are arrangedto transmit the frames of the second link protocol in place of theframes of the first link protocol or in the information fields thereofbetween the interworking unit and the mobile services switching centre,and the mobile station and the mobile services switching centre arearranged to use said data block numbering in the retransmissionmechanism of the second link protocol over an entire connection betweenthe mobile station and the mobile services switching centre.
 21. Amobile station configured to transmit and receive data in frames of alink protocol provided with a retransmission mechanism, wherein the datais placed in information fields of protocol frames in the form offixed-length data blocks which are numbered, and said retransmissionmechanism is arranged to utilize said data block numbering, and thelength of each protocol frame can be changed during a connection, andthe mobile station is arranged to insert the data blocks to beretransmitted into one or several protocol frames with a new framelength in response to the changing of the frame length, said data blockshaving been transmitted for the first time before the frame length waschanged and wherein the information field of each protocol framecomprises one or more data blocks and a header field of a protocol frameis provided with payload numbering indicating the data blocks containedin the information field of the protocol frame.
 22. A mobile stationaccording to claim 21, wherein the mobile station is arranged toacknowledge appropriately received data blocks, to request transmissionof new data blocks, or to request retransmission of inappropriatelyreceived data blocks.
 23. A mobile station according to claim 21,wherein said link protocol provided with a retransmission mechanism is alayer 2 link protocol or a protocol situated below a layer 2 linkprotocol.
 24. A mobile station configured to transmit and receive datain frames of a link protocol provided with a retransmission mechanism,wherein the data is placed in information fields of protocol frames inthe form of fixed-length data blocks which are numbered, and saidretransmission mechanism is arranged to utilize said data blocknumbering, and the length of each protocol frame can be changed during aconnection, and the mobile station is arranged to insert the data blocksto be retransmitted into one or several protocol frames with a new framelength in response to the changing of the frame length, said data blockshaving been transmitted for the first time before the frame length waschanced, the mobile station being a dual-mode mobile station withability to operate in two radio systems with different radio interfaces.25. A mobile station configured to transmit and receive data in framesof a link protocol provided with a retransmission mechanism, wherein thedata is placed in information fields of protocol frames in the form offixed-length data blocks which are numbered, and said retransmissionmechanism is arranged to utilize said data block numbering, and thelength of each protocol frame can be changed during a connection, andthe mobile station is arranged to insert the data blocks to beretransmitted into one or several protocol frames with a new framelength in response to the changing, of the frame length, said datablocks having been transmitted for the first time before the framelength was changed, and wherein the header of a protocol frame normallycontains the payload number of one payload unit situated in theinformation field, and the header of the protocol frame contains theindividual payload number of each payload unit in the information fieldwhen payload units with unsuccessive numbers are retransmitted in theprotocol frame in a special situation.